sched/signal: reclaim sigaction blocks

[yf13]

Summary

Currently the sigaction objects are never reclaimed after use, thus after running ostest we will see used memory increasion.

This patch uses a configurable number of pre-allocated entries and reclaims dynamic entries so that to reduce leakage.

Impact

Used memory reduced after running ostest.

Testing

• Checked with rv-virt/nsh and rv-virt/nsh64
• CI checks