MFC r265308:
If the traced process stops because it received a signal, libproc needs
to ensure that the signal is forwarded when proc_continue() is called.

MFC r256661 r257222 r257235 r257248 r257298.

MFC r256661:
Fix the libproc build when DEBUG is defined.

MFC r257222:
Clean up the debug printing in libproc a bit. In particular:

* Don't print any error messages to stderr unless DEBUG is defined.
* Add a DPRINTFX macro for use when errno isn't set.
* Print the error string from libelf when appropriate.

MFC r257235:
Remove an incorrect debug printf.

MFC r257248:
Fix the build with gcc.

MFC r257298:
Revert r257248 and fix the problem in a way that doesn't violate style(9).

MFC r265308:
If the traced process stops because it received a signal, libproc needs
to ensure that the signal is forwarded when proc_continue() is called.

MFC r256661 r257222 r257235 r257248 r257298.

MFC r256661:
Fix the libproc build when DEBUG is defined.

MFC r257222:
Clean up the debug printing in libproc a bit. In particular:

* Don't print any error messages to stderr unless DEBUG is defined.
* Add a DPRINTFX macro for use when errno isn't set.
* Print the error string from libelf when appropriate.

MFC r257235:
Remove an incorrect debug printf.

MFC r257248:
Fix the build with gcc.

MFC r257298:
Revert r257248 and fix the problem in a way that doesn't violate style(9).