This add a new pgp_armor_headers function to extract armor headers from an
ASCII-armored blob, and a new overloaded variant of the armor function, for
constructing an ASCII-armor with extra headers.

Marko Tiikkaja and me.

Instead of trying to accurately calculate the space needed, use a StringInfo
that's enlarged as needed. This is just moving things around currently - the
old code was not wrong - but this is in preparation for a patch that adds
support for extra armor headers, and would make the space calculation more
complicated.

Marko Tiikkaja

This includes removing tabs after periods in C comments, which was
applied to back branches, so this change should not effect backpatching.

pgp.h used to require including mbuf.h and px.h first.  Include those in
pgp.h, so that it can be used without prerequisites.  Remove mbuf.h
inclusions in .c files where mbuf.h features are not used
directly.  (px.h was always used.)

be more wary about having a value for BYTE_ORDER, clean up randomly-
chosen ways of including Postgres core headers.
Marko Kreen and Tom Lane