utils: provide safer `strncpy` function #255

Job	Run time
standalone-examples (bootstrap_server)	27s
standalone-examples (client/udp)	32s
standalone-examples (client/tinydtls)	29s
standalone-examples (client/raw_block1)	25s
standalone-examples (lightclient)	24s
standards (clang-18, 99, ON)	50s
standalone-examples (server)	55s
standards (clang-18, 99, OFF)	29s
standards (clang-18, 11, ON)	26s
standards (clang-18, 11, OFF)	1m 9s
standards (gcc-13, 99, ON)	35s
standards (gcc-13, 99, OFF)	1m 10s
standards (gcc-13, 11, ON)	26s
standards (gcc-13, 11, OFF)	28s
	8m 45s

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