Escape all minus characters in the man pages (#449)

Escape all minus characters in the man pages (#449)

From man-pages(7):

   Where a real minus character is required (e.g., for numbers such as -1,
   for man page cross references such as utf-8(7), or when writing options
   that  have a leading dash, such as in ls -l), use the following form in
   the man page source:

       \-

Signed-off-by: Marco d'Itri <md@linux.it>