regcomp.c: Use POSIXA, NPOSIXA

regcomp.c: Use POSIXA, NPOSIXA

This commit optimizes character classes which are matched under /a or
/aa and consist of a single Posix class, into POSIXA or NPOSIXA regop
types.  For example /[[:word:]]/a.  Since [:ascii:] is always
ascii-restricted no matter what the charset modifier is, it is always
optimized.

These nodes should execute somewhat faster than a generic ANYOF node,
and are significantly smaller, taking 2 bytes instead of 12.

The flags field of the node structure is used to hold an enum indicating
which of the 15 Posix classes is being matched.