Support standard Terminfo direct mode terminals

Support standard Terminfo direct mode terminals

Latest Terminfo introduces terminal definitions that support direct
color mode.  The "Co"/"colors" capability is set to 0x1000000 on these
terminals and Emacs is already compatible with them.
However, if used Terminfo library hasn't been compiled with 32-bit
value support, "Co"/"colors" is truncated to 0x7fff.  In this case
direct color mode support can be detected from the "RGB" capability
flag.
There are some minor problems if the color count isn't corrected from
0x7fff.  First eight standard colors defined in xterm-standard-colors
are shown correctly.  However, their RGB values match the terminal
settings, not the RGB values defined in xterm-standard-colors.  Bright
versions of these colors are shown incorrectly.  They are interpreted
as pixels #000008 - #000015, which are very dark shades of blue.

* src/term.c (init_tty): Force terminal color count to 0x1000000 if
"RGB" capability is present.
* src/tparam.h: Define prototype for tigetflag.  (Bug#30308)

* doc/misc/efaq.texi (Colors on a TTY): Add information about direct
mode terminals supported by Terminfo.