utils: provide safer `strncpy` function #250

Job	Run time
standalone-examples (bootstrap_server)	26s
standards (clang-18, 99, ON)	26s
standalone-examples (client/udp)	29s
standards (clang-18, 99, OFF)	32s
standalone-examples (client/tinydtls)	30s
standards (clang-18, 11, ON)	31s
standalone-examples (client/raw_block1)	27s
standards (clang-18, 11, OFF)	31s
standalone-examples (lightclient)	27s
standards (gcc-13, 99, ON)	27s
standalone-examples (server)	25s
standards (gcc-13, 99, OFF)	27s
standards (gcc-13, 11, ON)	25s
standards (gcc-13, 11, OFF)	2m 26s
	8m 29s

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