libquota: clean up some gcc -Wall warnings

libquota: clean up some gcc -Wall warnings

Remove unused variables, places where 'return' was used with no value
in a non-void function, missing function declarations, etc.  Don't
assume that all systems have quotactl(), and use <sys/quota.h> if it
exists to define the quotactl interfaces.

One of the unused variables also got rid of a non-portable use of
PATH_MAX.

Cc: Aditya Kali <adityakali@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>