Add the ability to correctly read pkg.conf is exists.

Only look for boostrap useful options:
 - PACKAGESITE
 - ABI
 - MIRROR_TYPE
 - ASSUME_ALWAYS_YES

While here makes PACKAGESITE expand the ${ABI} variable.
Allow to deactivate any SRV record look up (MIRROR_TYPE=none)
Use the same mechanism as for pkgng itself: first get configuration out of
environment variable and fallback on pkg.conf if exists.

Reviewed by:	bdrewery
Write lock is not required for find&compare operation.

MFC after:	2 weeks
choose in priority the allocated ABI if any to do the ${ABI} substitution in PACKAGESITE

Reviewed by:	bdrewery