CLEANUP: cfgparse: de-uglify early file error handling in readcfgfile()

CLEANUP: cfgparse: de-uglify early file error handling in readcfgfile()

In readcfgfile() when malloc() fails to allocate a buffer for the
config line, it currently says "parsing[<file>]: out of memory" while
the error is unrelated to the config file and may make one think it has
to do with the file's size. The second test (fopen() returning error)
needs to release the previously allocated line. Both directly return -1
which is not even documented as a valid error code for the function.

Let's simply make sure that the few variables freed at the end are
properly preset, and jump there upon error, after having displayed a
meaningful error message. Now at least we can get this:

  $ ./haproxy -f /dev/kmem
  [NOTICE] 116/191904 (23233) : haproxy version is 2.4-dev17-c3808c-13
  [NOTICE] 116/191904 (23233) : path to executable is ./haproxy
  [ALERT] 116/191904 (23233) : Could not open configuration file /dev/kmem : Permission denied