Log Message:
Update txt2man to 1.5.6.
PR 46535 by Wen Heping.
While here,
* set LICENSE=gnu-gpl-v2.
* honor PKGMANDIR

ChangeLog:
txt2man-1.5.6   16-mar-2011
* txt2man: Better control of 3 letters regexp, by Robin Cornelius.
* txt2man: Fix header comment generation, by Robin Cornelius.
* txt2man: Better handle ticks, by Thomas Moschny.
* src2man: Fix header comment generation.
* src2man: more robust comment delimiter handling.
* src2man: support C prototypes with newline after type, by Diego Cena.
* bookman: Posix shell syntax.