P1: restrict allowed headers in userspace code for p1 to allow for easier compilation in freestanding environment

[theyoyojo]

• motivation: compiler using libc headers
• allow e.g. only unistd.h and stdio.h, string.h, stdlib.h if that's all it takes
• verify same code builds with nolibc as include directory
• helpful to find out which student subs were fubar beforehand
• important for automation

[theyoyojo]

headers allowed: unistd.h, stdio.h, stdlib.h