Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Clean out some empty mergeinfo records, presumably by people doing local
cp/mv operations. The full repo-relative URL should be specified for the
source in these cases.

Add implementations of acosl(), asinl(), atanl(), atan2l(),
and cargl().

Reviewed by: bde
sparc64 testing resources from: remko

Implement carg(3) and cargf(3).

Rotting in an old src tree since: March 2005