Move renaming of local symbols after checking for branch instructions

This simplifies identifying local symbols within the branch
heuristic block.
Add an option for trying to automatically add a prefix to external symbols
Move the initial handling of lines into parse_line

This allows doing the same cleanup on lines that don't come from
the initial cpp invocation read in the initial while (<ASMFILE>)
loop.
Add initial support for .include

This misses support for checking include directories specified
with -I parameters.
Don't comment out the whole line when handling .type

libopus piles up multiple directives on one line.
Only interpret .if if it's the first word of a line

The arm2gnu.pl script in libopus converts an IF (from a license
header) into a .if, which messed up gas-pp.