Fix 0013227: Oracle database support (oci8)

Mantis 1.2.6 currently does not work with Oracle DB:

1. Installation:
   1.1. Oracle DB autocreates PK, so index creation for same field forbidden
   1.2. Oracle DB uses datetime literal format timestamp'YYYY-MM-DD HH-MI:SS'
   1.3. Oracle DB don't allows altering field property NOT NULL into NOT NULL
   1.4. Oracle DB max object length is 30 chars, so some index names must be reduced
   1.5. Oracle DB means empty string as NULLs, so NOT NULL restriction must be disabled for some field
   1.6. Oracle DB can resolve database server name through TNS, so database name cannot be required
2. General:
   2.2. Direct DB query execution result accessing instead of db_fetch_array() 1.3. usage didn't works with Oracle DB
   2.4. Oracle DB binds variable by name, so bind names in statement must be sorted to address them.
   2.5. Oracle DB handles NULL/DEFAULT values with specific way.
   2.6. Oracle DB returns NULL value as true PHP null
   2.7. Oracle DB handles sequence access with specific syntax
   2.8. Nothing returned by db_prepare_string() in case of oci8
   2.9. Oracle DB max object length is 30 chars, so table names must be reduced
   2.10. Oracle DB uses LOB literal format similar to mssql
   2.11. GetRowAssoc returns empty field values with oci8, it's need to enable returning both associative and indexed arrays.

The original patch was provided by DKuranov. He reckons that this also resolves
issues 0006853, 0007644, 0010437, 0010996, 0011265, 0011270, 0011276, 0012152, 0012478

Porting to 1.3 - Conflicts:
admin/install.php
admin/schema.php
core/database_api.php
manage_tags_page.php