Improve errExit error messages

Changes:

* Move msg to the end of errExit (right before perror(3p))
* Include the full file path (within the repository)
* Add "()" to function name for clarity

Before:

    Error malloc: main.c:123 main: Cannot allocate memory

After:

    Error src/firejail/main.c:123 main(): malloc: Cannot allocate memory

Note: This clarifies which is the exact file that the error message
comes from, as there are many source files with the same name.  For
example:

    $ git ls-files 'src/*/main.c' | wc -l
    20

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libtrace/Makefile b/src/libtrace/Makefile
index 8b14a4335..337529361 100644
--- a/src/libtrace/Makefile
+++ b/src/libtrace/Makefile
@@ -2,6 +2,7 @@
 ROOT = ../..
 -include $(ROOT)/config.mk
 
+MOD_DIR = src/libtrace
 SO = libtrace.so
 TARGET = $(SO)