quota: integrate mkquota.h into quotaio.h

quota: integrate mkquota.h into quotaio.h

There are interfaces that are used by mke2fs.c and tune2fs.c which are
in quotaio.h, and some future changes will be much simpler if we can
combine the two header files together.  Also the guard #ifdef for
mkquota.h was incorrect, which caused problems when both header files
needed to be included.

Also remove quota.pc and installation rules for libquota, since this
library is never going to be something that we can export externally
anyway.  Eventually we'll want to clean up the interfaces and move the
external publishable interfaces to the libext2fs library, and then
rename what's left from libquota.a to libsupport.a for internal use
only.

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