Improve conflict and nonconformance detection.

 * Conflicts are now recorded mutually, and headers that conflict once are
   checked for further conflicts.  Also many bugfixes.
 * errors.ini can now report "cautions", which are problems that are not
   severe enough to mark the header BUGGY, provided they don't occur in
   all of the two (or four) possible compilation modes.
 * errors.ini stanzas can apply to more than one header, or to all headers.
 * errors.ini is now aware of more system problems.

This is adequate to get a full inventory for glibc 2.17 out of single_test.py.
Add errors and prereqs for Solaris 11 in new pickier mode.

Also critical bugfix for the conflict detector (generate includes in
the appropriate mode).
Additional errors for FreeBSD 9.
Generalize 'use termios.h' error to apply to sgtty.h as well (FreeBSD 7).