Remove strdup calls from at_c

[XavierChanth]

• strdup is an extension in C99
• Instead use malloc, strlen, & memcpy to achieve the exact same thing

For portability, avoid using the extension.

[XavierChanth]

Marked as 0 SP as the remaining work will be completed via other changes, and the remaining places it exists is currently not an issue for current build targets (but may be for new ones)