Assigned 27 Sept; due 16 Oct.

Write a shell script (using either the Bourne shell /bin/sh or the
C shell /bin/csh) to keep several successive backup copies of a file.
Your script should be named "backup" and should take one parameter,
which is the name of an existing file, e.g. "backup foo.c".  It will
copy the file named "foo.c" to "foo.c.1", unless there is already a file
by that name, in which case it will try "foo.c.2", and so on up to
"foo.c.5".  If there are already files by all of these names, your
program should print an error message.

Extra credit: eliminate the built-in limit of 5, so your program can
keep as many old copies of the file as the operating system will allow,
e.g. "foo.c.158", "foo.c.2714", etc.

Example run:
panther% ls
foo.c	snark
panther% backup foo.c
panther% ls
foo.c	foo.c.1	snark
panther% backup foo.c
panther% ls
foo.c	foo.c.1	foo.c.2	snark
panther% backup foo.c
panther% ls
foo.c	foo.c.1	foo.c.2	foo.c.3	snark
panther% backup foo.c
panther% ls
foo.c	foo.c.1	foo.c.2	foo.c.3	foo.c.4	snark
panther% backup snark
panther% ls
foo.c	foo.c.1	foo.c.2	foo.c.3	foo.c.4	snark	snark.1
panther% backup foo.c
panther% ls
foo.c	foo.c.1	foo.c.2	foo.c.3	foo.c.4	foo.c.5	snark	snark.1
panther% backup foo.c
backup: There are already too many backup copies of foo.c.
panther%