dhcpd: Supports kernel mode compilation

[zs39]

Summary

The previous method of terminating the dhcp daemon via a global variable has been removed; instead, the kill pid command is used. Also, because the original dhcp_start method creates tasks via task_create, which is not supported in kernel mode, dhcp_start is not compiled in kernel mode. The daemon can be started in kernel mode using dhcpd xxx &.

Impact

The dhcpd_start and dhcpd_stop functions

Testing

Tested normally in flat mode

nsh> dhcpd_start eth0
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069568 Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 hpwork 0x400b8020 0x400b80a0
    2     0     0 100 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 lpwork 0x400b80e0 0x400b8160
    3     3     0 100 FIFO     Task      - Running            0000000000000000 0067488 nsh_main
    5     5     4 100 FIFO     Task      - Waiting  Semaphore 0000000000000000 0067464 dhcpd eth0
nsh> dhcpd_stop
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069568 Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 hpwork 0x400b8020 0x400b80a0
    2     0     0 100 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 lpwork 0x400b80e0 0x400b8160
    3     3     0 100 FIFO     Task      - Running            0000000000000000 0067488 nsh_main
nsh> 

The build process works correctly in kernel mode and also functions properly on the internal QEMU platform.

[xiaoxiang781216]

@zs39 please fix:

../nuttx/tools/checkpatch.sh -c -u -m -g 9da2bc06961587ec0d288686099a3e75524f8cc3..HEAD
Used config files:
    1: .codespellrc
/home/runner/work/nuttx-apps/nuttx-apps/apps/netutils/dhcpd/dhcpd.c:1019: re-use ==> reuse
/home/runner/work/nuttx-apps/nuttx-apps/apps/netutils/dhcpd/dhcpd.c:1407: re-used ==> reused
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