BUG/MINOR: ist: allocate nul byte on istdup

BUG/MINOR: ist: allocate nul byte on istdup

istdup() is documented as having the same behavior as strdup(). However,
it may cause confusion as it allocates a block of input length, without
an extra byte for \0 delimiter. This behavior is incoherent as in case
of an empty string however a single \0 is allocated.

This API inconsistency could cause a bug anywhere an IST is used as a
C-string after istdup() invocation. Currently, the only found issue is
with 'wait' CLI command using 'srv-unused'. This causes a buffer
overflow due to ist0() invocation after istdup() for be_name and
sv_name.

Backport should be done to all stable releases. Even if no bug has been
found outside of wait CLI implementation, it ensures the code is more
consistent on every releases.