libbladeRF: Check $HOME before using getpwuid() to determine home dir

Per the getpwuid() man page:

    An application that wants to determine its user's home  directory
    should  inspect  the  value of  HOME (rather  than  the  value
    getpwuid(getuid())->pw_dir) since this allows the user to modify their
    notion of "the home directory" during a login session.  To  determine
    the (initial) home directory of another user, it is necessary to use
    getpwnam("username")->pw_dir or similar.
libbladeRF: Comment and copyright date touch-up
libbladeRF: Removed extraneous padding character in find_file()

full_path is allocated with (PATH_MAX_LEN + 1) characters. Using
PATH_MAX_LEN as the max is sufficient to ensure the terminating '\0'
is left in place.
libbladeRF: Touched up calloc() call

Touched up calloc() invocation just for the sake of having the
nmemb & size parameters match.
libbladeRF: Handle file_size() being called with a directory handle

It appears that instead of returning -1, ftell() returns LONG_MAX
when called with a directory handle. file_size() has been updated
to check for this.