utils: provide safer `strncpy` function #256

Job	Run time
standards (clang-18, 99, ON)	28s
standalone-examples (bootstrap_server)	28s
standards (clang-18, 99, OFF)	50s
standards (clang-18, 11, ON)	29s
standalone-examples (client/udp)	29s
standards (clang-18, 11, OFF)	35s
standalone-examples (client/tinydtls)	28s
standalone-examples (client/raw_block1)	38s
standards (gcc-13, 99, ON)	30s
standalone-examples (lightclient)	25s
standalone-examples (server)	37s
standards (gcc-13, 99, OFF)	26s
standards (gcc-13, 11, ON)	33s
standards (gcc-13, 11, OFF)	27s
	7m 23s

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