* m4/absolute-header.m4: Renamed from full-header-path.m4.

* m4/absolute-header.m4: Renamed from full-header-path.m4.
This is to keep the terminology clean; POSIX talks about
"absolute pathnames", not "full pathnames", but the GNU
Coding Standards say to use "path" for something else;
so use "absolute" to keep both sides happy.
(gl_ABSOLUTE_HEADER): Renamed from gl_FULL_HEADER_PATH.
Set gl_absolute_header, not gl_full_header_path.
Set gl_cv_absolute_<header>, not gl_full_path_<header>.
Define ABSOLUTE_<HEADER>, not FULL_PATH_<HEADER>.
All uses changed.