Replaces (const char *file, unsigned int line, const char *function) arguments
to all logging functions, simplifies malloc/free tracking code in
overlay_buffer.c and Rhizome manifest alloc/free tracking in rhizome_bundle.c.
Use __HERE__ macro instead of (__FILE__, __LINE__, __FUNCTION__) everywhere.
Special __NOWHERE__ macro is equivalent to (NULL, 0, NULL).
Declare net.c functions in new "net.h" header, so log.c doesn't have to pull
in the entire "serval.h" just to use write_str().
Facilitates progress on issue #2.
Now write_str(), write_nonblock(), write_all(), set_nonblock() etc. report the
file/line/function of their caller, rather than the function in net.c. Done
using macros, similar in style to WHY() etc.
SET_NONBLOCKING(), SET_BLOCKING(), WRITE_STR() are now set_nonblock(),
set_block() and write_str() respectively, all of which log an error before
returning -1. There are other useful methods: write_all() treats anything less
than all bytes written as an error; write_nonblock() treats EAGAIN and EINTR as
zero bytes written, and a combination: write_all_nonblock().
