basic/process-util: only try PR_SET_MM once

basic/process-util: only try PR_SET_MM once

userwork wants to update the title many times, and a strace is full of
attempts that fail the same way:

[pid 21765] prctl(PR_SET_NAME, "systemd-userwor"...) = 0
[pid 21765] geteuid()                   = 0
[pid 21765] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fedce329000
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_START, 0x7fedce329000, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_END, 0x7fedce32901d, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] munmap(0x7fedce329000, 4096) = 0
[pid 21765] accept4(3, NULL, NULL, SOCK_CLOEXEC|SOCK_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
[pid 21765] prctl(PR_SET_NAME, "systemd-userwor"...) = 0
[pid 21765] geteuid()                   = 0
[pid 21765] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fedce329000
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_START, 0x7fedce329000, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_END, 0x7fedce329020, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] munmap(0x7fedce329000, 4096) = 0
[pid 21765] prctl(PR_SET_NAME, "systemd-userwor"...) = 0
[pid 21765] geteuid()                   = 0
[pid 21765] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fedce329000
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_START, 0x7fedce329000, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_END, 0x7fedce32901d, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] munmap(0x7fedce329000, 4096) = 0
[pid 21765] accept4(3, NULL, NULL, SOCK_CLOEXEC|SOCK_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
[pid 21765] prctl(PR_SET_NAME, "systemd-userwor"...) = 0
[pid 21765] geteuid()                   = 0
[pid 21765] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fedce329000
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_START, 0x7fedce329000, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_END, 0x7fedce329020, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] munmap(0x7fedce329000, 4096) = 0
[pid 21765] prctl(PR_SET_NAME, "systemd-userwor"...) = 0
[pid 21765] geteuid()                   = 0
[pid 21765] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fedce329000
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_START, 0x7fedce329000, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_END, 0x7fedce32901d, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] munmap(0x7fedce329000, 4096) = 0
[pid 21765] accept4(3, NULL, NULL, SOCK_CLOEXEC|SOCK_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
[pid 21765] prctl(PR_SET_NAME, "systemd-userwor"...) = 0
[pid 21765] geteuid()                   = 0
[pid 21765] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fedce329000
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_START, 0x7fedce329000, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] prctl(PR_SET_MM, PR_SET_MM_ARG_END, 0x7fedce329020, 0, 0) = -1 EPERM (Operation not permitted)
[pid 21765] munmap(0x7fedce329000, 4096) = 0

If we get a permission error, don't try again.