Autoconf cleanup.

Use standard macro for detecting strtod.  Add detection of grep and
remove uses of grep -q (bad for Solaris, thanks Maciej).  Remove
detection of C++, uint16_t, several functions and headers that
are not used and some other cruft.

Shaves several seconds off ./autogen.sh.
Speed up travis.

Don't call configure twice.  Parallelize makes.
Change to the agreed API prefix.

Switched from text_format to the more consistent protobuf_c_text.
Also tweaked some docs in the process.
Move doxygen and code coverage docs.