Several fixes for DragonFly
On DragonFly use procfs for self_exe_name()

Using /proc/curproc/file does not resolve mount-points as sysctl(3) with
KERN_PROC_PATHNAME does, which gave back paths containing colons like
/pfs/@@-1:00004/usr/bin. This led to problems when joining paths based on it.
DragonFly 4.0 linker path changed to lib/gcc48
Merge pull request #19852 from mneumann/dragonfly3

Several fixes for DragonFly

Reviewed-by: brson