Put back whole-file #ifdefs for all files that are conditionally built.

This was apparently an intentional addition for the sake of IDEs rather
than something held over from a less capable build system.
Small improvements to the handling of strlcpy.

Code inside libevent should now always use 'event_strlcpy', which
strlcpy-internal.h will #define as 'strlcpy' if the platform has it, or
'event_strlcpy_' (note trailing underscore) if it doesn't.  Also modernize
function definition within strlcpy.c.