login,libblkid: use econf_readConfig rather than deprecated econf_readDirs

login,libblkid: use econf_readConfig rather than deprecated econf_readDirs

Fixes:

lib/logindefs.c: In function ‘load_defaults’:
lib/logindefs.c:257:9: warning: ‘econf_readDirs’ is deprecated: Use the econf_readConfig/econf_readConfigWithCallback instead [-Wdeprecated-declarations]
  257 |         error = econf_readDirs(&file,
      |         ^~~~~

libblkid/src/config.c: In function 'blkid_read_config':
libblkid/src/config.c:164:17: error: 'econf_readDirs' is deprecated: Use the econf_readConfig/econf_readConfigWithCallback instead [-Werror=deprecated-declarations]
  164 |                 error = econf_readDirs(&file,
      |                 ^~~~~

* check for econf_readConfig() by ./configure and moson

* add UL_VENDORDIR_PATH into pathnames.h to avoid #ifdef in code

* use #ifdef HAVE_ECONF_READCONFIG to switch between econf_readDirs()
  and the econf_readConfig()

Signed-off-by: Karel Zak <kzak@redhat.com>