Get rid of LIBSTOKEN_HDR and LIBOATH_HDR

Unlike libproxy, these two libraries have well-defined names for their
respective header files.  So include the headers by name, and use
HAVE_LIBSTOKEN / HAVE_LIBOATH for compile-time tests.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Fix token-related command line options

Aliasing --stoken to --token-secret is not effective because token_mode
does not get set.  Might as well just drop --stoken.

For --token-mode, change "stoken" to "rsa" to make the UI more intuitive:
liboath provides TOTP token support, and libstoken provides RSA token
support.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Update man page and changelog to reflect --token-* changes

Also, tweak the man page syntax so it is more consistent with how other
options are documented.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>