This adds the per-thread global variable git_errno to the
system, which callers can examine to get information about
an error.

Two helper functions are added to reduce LoC-count for the
library code itself.

Also, some exceptions are made for running sparse on GIT_TLS
definitions, since it doesn't grok thread-local variables at
all.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>