optimize config_read() (by Timo Teras <timo.teras AT iki.fi>)

function                                             old     new   delta
bb_get_chunk_with_continuation                         -     176    +176
find_pair                                            169     187     +18
...
process_stdin                                        443     433     -10
config_read                                          549     456     -93
bb_get_chunk_from_file                               139       7    -132
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 7/7 up/down: 215/-254)          Total: -39 bytes

diff --git a/include/libbb.h b/include/libbb.h
index 075fa05..388e2f9 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -613,6 +613,7 @@
 extern void xprint_and_close_file(FILE *file) FAST_FUNC;
 
 extern char *bb_get_chunk_from_file(FILE *file, int *end) FAST_FUNC;
+extern char *bb_get_chunk_with_continuation(FILE *file, int *end, int *lineno) FAST_FUNC;
 /* Reads up to (and including) TERMINATING_STRING: */
 extern char *xmalloc_fgets_str(FILE *file, const char *terminating_string) FAST_FUNC;
 /* Chops off TERMINATING_STRING from the end: */