MDEV-452 Add full support for auto-initialized/updated timestamp and datetime

Generalized support for auto-updated and/or auto-initialized timestamp
and datetime columns. This patch is a reimplementation of MySQL's
"WL#5874: CURRENT_TIMESTAMP as DEFAULT for DATETIME columns". In order to
ease future merges, this implementation reused few function and variable
names from MySQL's patch, however the implementation is quite different.

TODO:
The only unresolved problem in this patch is the semantics of LOAD DATA for
TIMESTAMP and DATETIME columns in the cases when there are missing or NULL
columns. I couldn't fully comprehend the logic behind MySQL's behavior and
its relationship with their own documentation, so I left the results to be
more consistent with all other LOAD cases.

The problematic test cases can be seen by running the test file function_defaults,
and observing the test case differences. Those were left on purpose for discussion.

1 files changed, 1 insertions, 0 deletions

diff --git a/sql/sql_string.h b/sql/sql_string.h
index 2f0cd9103dc..633170a5e20 100644
--- a/sql/sql_string.h
+++ b/sql/sql_string.h
@@ -343,6 +343,7 @@ public:
   bool append(IO_CACHE* file, uint32 arg_length);
   bool append_with_prefill(const char *s, uint32 arg_length, 
 			   uint32 full_length, char fill_char);
+  bool append_parenthesized(long nr, int radix= 10);
   int strstr(const String &search,uint32 offset=0); // Returns offset to substring or -1
   int strrstr(const String &search,uint32 offset=0); // Returns offset to substring or -1
   bool replace(uint32 offset,uint32 arg_length,const char *to,uint32 length);