Add error message when child exits whit error

Master proccess was not showing any error message when the child
died with an error, and it was very confusing for the user (for
example with incorrect -e command).
Unset mode when clearing regions

tclearregion() was clearing regions using spaces and the current
attributes of the terminal. It was correct with all the modes excepct
underline, because they didn't affect the space character, but in
the case of underline it was a problem. A easy way of seeing this
problem is writing this in the last line of the terminal:

	tput smul ; echo first; tput rmul; echo second; echo third

Fist was underlined, and second and third were not underlined, but
the spaces at the right of second was underlined becuause in the
previous scrool underline mode was set.
Remove ul capability

This capability indicates that underscore '_' overstrike current
letter under the cursor. It means that you can generate a
underline 'b' using 'b^H_', because it writes a 'b' then backward
one characther and then overstrike '_'. St has not such behaviour,
so it is an error to have this capability.
Merge branch 'master' of http://git.suckless.org/st