Major cleanup; may imply minor changes in edge cases of error reporting.

Major cleanup; may imply minor changes in edge cases of error reporting.

Finally, drop support for the run-time configurable mandocmsg()
callback.  It was over-engineered from the start, never used for
anything in a decade, and repeatedly caused maintenance headaches.

Consolidate reporting infrastructure into two files, mandoc.h and
mandoc_msg.c, mopping up the bits and pieces that were scattered
around main.c, read.c, mandoc_parse.h, libmandoc.h, the prototypes
of four parsing-related functions, and both parser structs.