Change STRING length from int8_t to uint8_t.

This fixes GCC warning when using DELETE function from Standard Library.

In VARIABLES.CSV, RETAIN variable were only flagged as such for GLOBALS, now local POU variable shoud be as well.

Fixed logic for REPEAT..UNTIL

In Stage4, C code generator, REPEAT statement was generating a while loop with inverted logic

Added RETAIN qualifier to VARIABLES.CSV

Variable forcing now uses limited list and buffer instead of systematical instance tree traversal and in-tree "fvalue" to keep track of forced value for pointed variables (external, located). Pointer swapping is performed when forcing externals and located, with backup being restored when forcing is reset. Match Beremiz commit 93ad018fb602.

Fixed __SET_EXTERNAL_FB declaration in accessor.h, invoked when accessing EXTERNAL_BLOCK.VARIABLE. Macro was invoking __SET_VAR with wrong parameter count. Fixed corresponding code generator for ST. Seems similar codegen for IL is also broken, but fix would affect other (unknown) behviours -> didn't touch it.

Remove RETAIN qualifier for F_TRIG and R_TRIG 'M' variable. This is not following standard, and implicitely consumes a lot of RETAIN memory. User can explicitely qualify R_TRIG of F_TRIG instance as RETAIN if needed.

Fix in standard function library: forgot static linkage for inline __str_cmp()