Assigned 27 Oct, due 10 Nov.

Write a program, in C, named "wsp" (this stands for "where search path").
Your program will take two command-line arguments.  The first is the name
of an environment variable whose value is a list of directories, separated
by colons (e.g. PATH); the second is the name of a file.  Your program will
look in each of the directories in turn, find out whether that directory
contains a readable file with the specified name, and print the results on
the screen in some friendly form.  

Your program should handle errors (e.g.  wrong number of arguments, first
argument not the name of an environment variable, nonexistent directory,
etc.) gracefully, printing an informative error message rather than
crashing.  The list of directories may contain absolute pathnames, relative
pathnames, and "." and ".." notation; it will not contain "~" and "~userid"
notation.

Example: 
panther% ls
snark		dir1		dir2
panther% ls dir1
thisfile	thatfile	the_otherfile
panther% ls dir2
thisfile	thatfile	dir3
panther% ls dir2/dir3
thatfile	snark		boojum		words
panther% ls /usr/share/dict
README		british		hlista		hstop		words
american	hlist		hlistb		stop
panther% setenv DICTPATH dir2/dir3:/usr/share/dict:.
panther% wsp DICTPATH words
dir2/dir3/words
/usr/share/dict/words
panther% wsp DICTPATH snark
dir2/dir3/snark
./snark
panther% wsp DICTPATH american
/usr/share/dict/american
panther% wsp DICTPATH thisfile
"thisfile" not found
panther% setenv DICTPATH dir2/dir3:/usr/share/dict:dir1:.
panther% wsp DICTPATH thisfile
dir1/thisfile

EXTRA CREDIT 1: Some users may mistakenly type "wsp $DICTPATH snark" instead of
"wsp DICTPATH snark".  Write your program so that it still works in this case.

EXTRA CREDIT 2: Allow two optional flags "-f" and "-q", which may be combined in
any order ("-f -q", "-q -f", "-fq", or "-qf"), and which may only appear before
the arguments.  If the "-f" flag is provided, the program only prints out the
FIRST match it finds, rather than all matches; if the "-q" flag is provided and
the program finds nothing at all, it prints nothing at all rather than a "not
found" message.

EXTRA CREDIT 3: Handle the "~" and "~userid" notation correctly if they appear
in directory names in the path.  (The only way I can think of to do this is to
look them up in the passwd database, which would be fairly difficult in a C
program.  But maybe there's an easier way.)

HINTS: The way I envision writing this program, you'll need to use argc and
argv, the "getenv" library function, and the "strchr", "strcat", "strcpy", and
related library functions.  To find out whether a file with a specified name
exists in a specified directory and is readable, I suggest using "fopen" and
trying to open the file for read access.  If so, you should immediately close
it again, so as not to leave a lot of open files lying around.